The overall objective of this research project is the greater understanding of structures and functions of the photoreceptive rod outer segments (ROSs) of the vertebrate retina. In particular, we seek 1) to establish the distribution of molecular species in ROSs using x-ray scattering, freeze-fracture, and conventional electron microscopic techniques; and 2) to investigate changes in such distributions associated with a) the bleaching of rhodopsin at physiological light levels, and b) changes in external pH, ion concentrations and osmotic strength. We are using a combination of isopycnic centrifugation, low-angle x-ray diffraction and osmotic manipulation procedures on isolated ROSs to accurately establish the concentration of aqueous and non-aqueous components in the frog ROS. High-speed freezing of ROSs combined with low-temperature x-ray scattering and freeze-fracture replication will allow us to assess the relevance of freeze-fracture images to in vivo ROS disk membrane structure. X-ray scattering from isolated ROSs and intact retina specimens will be recorded to S equals (1/10A); the derived electron density profiles will be established on an absolute scale. Quantitative physiological bleaching studies of the ROSs will be carried out, and the changes in electron density identified.